Stargazing, Astronomy, and Night Sky Info

The Crab Nebula

Nearly a thousand years ago, on a late-April morning in the year 1054, groggy Chinese astronomers awakened to the spectacle of a blazing new star in the daytime sky. This “guest star”, as they called it, appeared out of nowhere, shone with a reddish-white light some six times brighter than Venus, then slowly faded from daytime view after 23 days. What they observed, of course, was a supernova, a massive star suddenly collapsed upon itself before exploding with as much energy as an entire galaxy.

European skywatchers must have observed this event also, but they were preoccupied with surviving the Dark Ages and made no record of this supernova. But the Chinese, along with their Japanese and Arabic colleagues, noted the position of this exploding star with enough precision that modern astronomers link this supernova with the “Crab Nebula”, a faint splatter of light easily seen in the constellation Taurus with binoculars or a small telescope.

The Crab Nebula is the first object in Messier’s famous catalog, so it’s commonly known as M1. It’s the only supernova remnant in the catalog. In a small telescope, the nebula appears as an oval splotch about 6’x4′ just one degree northwest of 3rd magnitude zeta Tauri (see map below). This makes it easy to find. At magnitude 8.4, it’s visible in 7×50 binoculars in dark sky. At 30x in a small telescope, the nebula fits in the same field of view as zeta Tauri. At higher-power, the nebula reveals a pinched off region near its centre. The delicate tendrils visible in images are visible only with difficulty in larger telescopes.

Astronomers know the Crab was created by a core-collapse supernova because the event left a pulsar, a dense city-sized remnant of the stellar core that spins around once every 33 milliseconds. The 16th magnitude pulsar can be seen visually with a 20-inch or larger telescope. But astronomers learn much more about this object through observations at radio and X-ray wavelengths.

Around the pulsar is a dense bubble of material bound by a strong magnetic field. Beyond that is the material ejected by the supernova itself. That’s what you see with your telescope. The remnant continues to expand outward at 1500 km/s, fast enough to notice during a human lifetime. The Hubble Telescope has revealed changes in the outer shell of the Crab Nebula over a few days, which is amazing for an object that’s some 6,000 light years away!

Click on image to see animation showing expansion of Crab Nebula (Credit: NOAO)

The nebula takes its name from a drawing made by Lord Rosse in 1844 with a 36-inch telescope at Birr Castle in Ireland. The drawing resembles a horseshoe crab.

Since M1 lies about 1.5 degrees off ecliptic, it’s sometime occulted by the Moon, which helps astronomers map the X-ray emission from the central region of the nebula. The Sun’s corona also passes in front of M1 every June, and the X-rays from the nebula help astronomers infer the physical nature of the corona. Astronomers also used the space-based Chandra X-ray observatory to observe Saturn’s moon Titan passing in front of the Crab. This helped determine the thickness of Titan’s atmosphere.